Multi-unit home networking setup

I have a house with three units but I only have one DSL connection available. What would be the smartest and cheapest solution to having three independent networks that all unify at the DSL connection/main router? I have a little networking experience but am willing to read up on things that need to be known. I also have a eve-ng server set up for simulating as I don't want to buy hardware before knowing how to solve it.

What router do I need for this?